function [FFv] = FormFunction(k,a)
%
%  function [FFv] = FormFunction(k,a)
%   
%  Input: k = acoustic wavenumber (=2*pi/lambda, lambda=c/f)
%         a = sediment radius (=d/2) in m
%
%  Output: FFv = form function matrix. No of rows = length(k)
%                                      No of cols = length(a)
%
%   calculates the FormFunction for sediment of radius a with 
%   sound of acoustic wavenumber k using Vincent, 1995
%    
%   Chris Vincent, UEA
%   Modified by G.V. February 2004
k=k(:);
a=a(:);
Na=length(a);
Nk=length(k);
%
k1=1.31;
k2=0.53; 
k3=1.41; 
k4=1.31;
%
for i=1:Na;   % For each particle size
    x=k*a(i);
    x2=x.*x;
    x5=x.^5;
    x28=x.^2.8;
    xk10=(k2.*x).^10;
    c3=(x./k3).^2;
    c4=(1+k1.*x)./(1+(k4.*x).^3);
    c5=(1+2.1*xk10)./(1+xk10);
    c6=(1.6+(x*2.5).^7)./(1+(x*2.5).^7);
    FFv(:,i)=c3.*c4.*c5.*c6/0.67;
end